Description of the Prior Art
This invention relates to an improvement of a de-salting method by electrodialysis.
A method which comprises de-salting e.g. molasses, fermented liquors etc. by electrodialysis in order to effectively utilize sources is well known. Heretofore, it has been known that when an electrodialysis method is applied to these liquors to be treated, ion exchange membranes, especially anion exchange membranes, are fouled, and their useful life is reduced. In order to avoid this drawback, it was necessary to conduct pretreatments, for example, decoloration with active carbon (Seito Gijutsu Kaishi, Vol. 28, p. 15 (1980)), treatment with an anion exchange resin (Japanese Patent Application Laid-open No. 67093/1979) etc. In other words, such treatments involved a number of steps and required expensive auxiliary materials, and as a result, they had a disadvantage of the expensive cost.
This invention presents a novel method for conducting de-salting of a liquor containing anion exchange membrane-fouling substances by electrodialysis without fouling the membranes.
The present inventors have been extensively studying for the purpose of attaining this object, and have come to discover that anion exchange membrane-fouling substances are mostly ampholytic polymers such as proteins, dyes, humic substances etc., and by appropriately adjusting the pH of the liquor, these substances can be converted to a non-charged state and thus the affinity to the membrane can be reduced, thereby having accomplished this invention.
Accordingly, this invention is a method for preventing fouling of an electrodialysis membrane when conducting desalting of an inorganic salt-containing aqueous solution containing electrodialysis membrane-fouling substances such as proteins, dyes, humic substances, etc. by electrodialysis, which is characterized by previously adjusting the pH of the solution to the isoelectric points of said fouling substances or the vicinity thereof.
Examples of the inorganic salt-containing aqueous solution containing electrodialysis membrane-fouling substances such as proteins, dyes, humic substances etc. include molasses such as cane molasses, beet molasses etc. dairy product waste liquors such as cheese whey etc., fermented liquors such as amino acids, nucleic acids etc. and their in-process liquors, protein hydrolysate liquors such as soybean source or its analogs etc.
The electrodialysis in this invention may be conventional in regard to having anion exchange resin membranes and cation exchange resin membranes arranged alternately and in that a voltage gradient is applied. The anion exchange resin membranes to be treated for the prevention of fouling are, for example, conventional anion exchange resin membranes which comprise a polymer having a styrenic backbone provided with a quaternary amine as functional groups, representative examples of this type being
AMV (produced by Asahi Glass Co.),
AF-4T, ACH-45T (produced by Tokuyama Soda Co.),
A-101 (produced by Asahi Chemical Industry Co.) etc.
The so-called membrane-fouling substances contained in the aforesaid liquors to be treated are mostly ampholytic polymers such as proteins, dyes, humic substances etc. which can be converted to a non-charged state by adjusting the pH of the liquor to their isoelectric points.
The adjustment of the pH may be made by employing an acid or alkali, examples of the acid which may be employed are mineral acids such as hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid etc. and organic acids such as acetic acid etc., and examples of the alkali include sodium hydroxide, potassium hydroxide, lime, ammonia etc.
Actual operations of this invention are described below.
The isoelectric points of the anion membrane-fouling substances contained in the aforesaid liquors to be treated are all known to be in the range of pH 3-4. According to this invention, it is satisfactory merely to adjust the pH of the aforesaid liquor to be treated with an acid or alkali to pH 3-4 and thereafter conduct de-salting by electrodialysis. By this, fouling of the membranes may be prevented by simple operations without employing active carbon or an anion exchange resin.